The Gaohu and the Zhonghu are common doubles for Erhu players. The Gaohu is higher pitched and the Zhonghu is the alto version. This instrument first appears after 1104 in the Song Dynasty. The Gaohu is particularly used in Southern China. Traditionally a Gaohu had two silk strings, but today metal strings or a combination of metal/silk strings are more common. The physical structure of a Gaohu is similar to that of an erhu, although the neck of this instrument is shorter. The tuning is a fourth to a fifth higher than the erhu, giving it a higher pitch voice. Because of its mellow tone quality, the Gaohu is chiefly used in Cantonese music. It can be used as a solo instrument normally for lively and merry music pieces.